Impedance Study of Thermal Barrier Coating Deterioration in Gas Turbines

Abstract

Some inherent characteristics of thermal barrier coatings (TBC), such as porosity, thinner thickness and relatively high electrical resistivity, limit the application of traditional room temperature non-destructive testing techniques.Therefore, a method for monitoring oxide growth in thermal barrier coatings using impedance spectroscopy in high-temperature environments was proposed.Based on the principle of impedance spectroscopy test, the growth information of oxides in thermal barrier coatings was obtained by analyzing the response of AC impedance in different frequency domains and phase angle information, and the prediction model was constructed based on the micromorphology of SEM.Results showed that after oxidizing at 1 000 ℃for different times, a dense dark thermally grown oxide (TGO) layer was formed between the ceramic layer and the adhesive layer, and its thickness increased with the prolongation of oxidation time, exhibiting a non-linear relationship.As the oxidation time prolonged, the composition of the TGO layer transitions from Al2O3 in the early stage of oxidation to a mixed oxide of Al2O3 (mainly), Cr2O3, CoO and NiO.Furthermore, the relationship between the resistance of the TGO layer and the oxidation time was constructed by establishing an appropriate circuit model to fit the impedance spectrum data, which confirmed the feasibility of non-destructive testing using impedance testing technology in the process of high-temperature degradation of thermal barrier coatings.

Keywords

• ac impedance； thermal barrier coating； actual machine temperature； deterioration process； nondestructive testing